Generally, acoustic transducers convert received electrical signals to acoustic signals when operating in a transmit mode, and/or convert received acoustic signals to electrical signals when operating in a receive mode. The functional relationship between the electrical and acoustic signals of an acoustic transducer depends, in part, on the acoustic transducer's operating parameters, such as natural or resonant frequency, acoustic receive sensitivity, acoustic transmit output power and the like.
A horn is an acoustic waveguide which provides an efficient means of coupling a sound source to the environment. Generally, horns may be used to amplify acoustic waves, as indicated by incorporation of horns in various acoustic devices such as loudspeakers and musical instrument, for example, to increase their sound output. Sound (acoustic waves) enters the horn at the throat and exits the horn at the mouth. In addition, a horn may be used to modify directionality characteristics or radiation pattern of an acoustic emitter, e.g., by the location, size and shape of the horn.
By extension, acoustic horns may be used with micromachined acoustic transducers, such as such as piezoelectric ultrasonic transducers and micro micro-electro-mechanical system (MEMS) transducers. When implemented on a small scale, an acoustic horn may be etched into a silicon substrate, for example, using a wet etchant, such as potassium hydroxide (KOH), which etches silicon preferentially along various crystal planes. However, KOH is an anisotropic etching process which produces limited results with respect to horn characteristics, as shown in the schematic diagram of FIG. 1. In particular, KOH etching produces a pyramidic shaped horn 110 in a substrate 105. The horn 110 has linear cross-sections and a square mouth, opening on an outside (e.g., top) surface of the substrate 105. Notably, an angle 112 defined by the mouth of the horn 110 and the outer surface of the substrate 105 is necessarily fixed at 54.7 degrees. These limitations on size and shape of micro machined acoustic horns constrain design flexibility.